The present invention is directed to an improved process for the production of polyester resin.
Aromatic polyester resins employable in applications such as molding, extrusion, injection and similar operations require relatively high molecular weights corresponding to intrinsic viscosity (IV) values generally higher than 0.65-0.75 dl/g.
The resins for film and fibre, on the contrary, have lower IV values, between about 0.6 and 0.75 dl/g.
The preparation of the resins is typically carried out by polycondensation of an aromatic dicarboxylic acid, normally terephthalic acid or its alkyl diesters with an aliphatic glycol operating under temperature and pressure conditions such as to obtain a resin with IV values as high as possible. It is difficult, however, to reach IV values above 0.65-0.75 dl/g because of the high melt viscosity which prevents an efficient removal of the byproducts of reaction.
Therefore, the reaction of polycondensation in a melt state (MPC) is carried out under high vacuum to remove the reaction byproducts.
The MPC polycondensation, however, is an expensive operation which is desirable to avoid.
The resin to be used for molding and similar operations, after the MPC stage, is then subjected to polycondensation treatment in solid state (SSP) with the aim of increasing the IV to the desired values (0.75-1.2 dl/g).
Prior to the SSP treatment, the resin granules are subjected to a crystallization treatment directed at increasing as much as possible the crystallinity of the polymer so as to avoid in the subsequent SSP step packing and sticking of the granules which under severe conditions can lead to plant stoppage.
The SSP step, however, requires a relatively long time (from several to 10 or more hours depending on the final IV value to be obtained).
Working in the MPC step with non high melt viscosity values and therefore with the IV of polymer relatively low, it is possible to more easily remove the reaction byproducts and to lower in this way its duration. The increase in IV to desired values could thereafter be obtained by SSP.
However, there exist limits in decreasing of melt viscosity mainly due to the presence of a large quantity of olygomers which are formed when operating under such conditions. The olygomers, in the next SSP step, cause the formation of cyclic compounds whose presence has a negative influence on the flowability of granules and therefore the regularity of the SSP operation.
With the aim of overcoming the above inconveniences it was proposed in U.S. Pat. No. 5,376,734 to add, with the resins having an IV lower than 0.57 dl/g, a dianhydride of a tetracarboxylic acid, for example pyromellitic dianhydride (PMDA), and to conduct the SSP reaction in the presence of such a dianhydride.
The IV values of the resins with which the dianhydride is added are not in practice lower than 0.4 dl/g (in the examples 0.408 dl/g). After addition of the dianhydride, the resin is pelletized by conventional systems and then has to be subjected to crystallization with the aim of being able to carry out the SSP treatment.